The invention relates in general to the field of sensors of driver assistance systems. In particular the invention relates to the correct detection of the temperature of ambient air for determining the current sound velocity, preferably for a distance measurement by means of ultrasound sensors on a motor vehicle.
For assisting a driver of the vehicle when maneuvering a vehicle, in particular parking and leaving a parking place, modern vehicles are equipped with a so-called parking system. Such parking systems are associated with the driver assistance system as parking assistance systems. Parking systems are usually based on distance measurements with ultrasound sensors and monitor a region of approximately 20 to 250 cm behind and, as the case may be, in front of the vehicle. An ultrasound sensor consists, for example, of an aluminum membrane, on the inner face of which a piezoceramic oscillator in the form of a thin disc for transmission and reception of ultrasonic signals is fastened. The electronics necessary for control and signal processing is generally located on a small circuit board screened off in a sensor housing. DE 198 16 456 C1 shows an example of such an ultrasound sensor.
In ultrasonic distance measurement the distance of the vehicle from an obstacle is ascertained according to the echo sounding principle from the propagation time between transmission and reception and also on the basis of the propagation speed of sound in the propagation medium (in this case in the ambient air). The ascertained distance is indicated to the driver optically and/or acoustically for orientation and/or warning.cΣ=(331.5+0.596·Σ)m/s. 
Thus, for correctly ascertaining the distance by means of ultrasound sensors an exact determination of the external temperature is necessary.
DE 100 20 958 A1 shows for example the carrying out of a corresponding temperature compensation for a plurality of ultrasound sensors centrally in a control device which is connected to a temperature sensor of the vehicle.
Conventional temperature measurement sensors have a precision of less than 1° C., but because of the installation location in the vehicle, problems occur in the determination of the correct air temperature outside the vehicle. The nearer the external temperature sensor is installed relative to heat sources, such as for example the engine or the exhaust gas system or a brake of the vehicle, the greater is the deviation of the actual air temperature and a measured value from the temperature sensor. Irrespective of the installation location, a vehicle which is stationary or travelling at low speed heats up. Usual reasons for this are for example sunlight falling on the vehicle and/or thermal convection from the ground surface, for example hot asphalt, and/or waste heat from the engine or the exhaust gas system. Therefore, when stationary or travelling slowly a correct determination of the external temperature by means of the temperature sensor is no longer possible. At higher vehicle speeds, for example above 80 km/h, these heating effects do not occur because of the wind resistance.
Therefore, the temperature sensor is preferably integrated at a location in the vehicle which is exposed to the wind resistance and a measured value from the temperature sensor is usually used only above a predetermined vehicle speed. At low speeds, the last temperature measured at higher speeds is used, and is maintained for a variable holding time (generally several hours).
The measurement of lower temperatures at lower speeds constitutes an exception. Since in this case heating effects play no part and the driver should be warned as early as possible of the danger of slippery conditions, for example at approximately 3° C. and below, measured temperature values which are lower than the previously applicable external temperature value, irrespective of the speed of travel, are immediately adopted.
However, for a parking assistance system an exact determination of the temperature of the ambient air, especially at low vehicle speeds, i.e., less than 5 km/h, is necessary in order to correctly ascertain the distance. As explained above, in this case often only “maintained” measured temperature values are available. In various situations, this may lead to an incorrect external temperature being assumed. For example, when travelling from a cold environment, such as a car park or an underground garage or a garage, into a significantly warmer environment, when starting the engine in a warm environment, when driving through a cold tunnel comprising with subsequent travel into the warmer environment outside the tunnel, when driving in the mountains with a temperature gradient, to name only a few cases which are relevant in practice.
The more the external temperature value used for the ultrasound-based distance measurement deviates from the real temperature of the ambient air, the greater the error in the distance calculation is.
A method for temperature compensation in a distance measurement with ultrasound is known from DE 10 2004 016 267 A1, wherein a distance from an obstacle is determined independently of the temperature solely from the propagation times measured in each case by the at least three ultrasound sensors and the data from the three-dimensional arrangement of the sensors relative to one another, because in the calculation of the distance the sound velocity and thus the temperature-dependent value is reduced. However, in this method a suitable environment or a test obstacle is necessary for the calibration of the system.
It is a possible object of the present invention to propose an improved method for distance measurement by means of ultrasound sensors for use in a vehicle, as well as a correspondingly improved distance measurement system, in particular a parking assistance system improved by means of temperature compensation even at low speeds.
This object is achieved by the features of the independent claims. Further features and details of the invention are apparent from the subordinate claims, the description and the drawings.
In this case, features and details which are described in connection with the driver assistance system according to the invention also of course apply in connection with a corresponding method and in each case vice versa, so that with respect to the disclosure reference is or can be always made reciprocally to the individual aspects.
A fundamental idea underlying the invention resides in an improved determination of the external temperature of a vehicle, i.e., the air temperature outside the vehicle, particularly in the low speed range. Within the context of the invention it has been recognized amongst other things that on this basis for example the distinction from obstacles/objects in the case of ultrasound-based driver assistance systems can be ascertained significantly more precisely and reliably. The invention is suitable in particular for use in the low speed range for parking assistance systems. Furthermore, the driver can also be informed better and more quickly by means of the current external temperature in the entire speed range.
A first aspect of the invention relates to a temperature measuring device for measurement of the external temperature of the ambient air in a vehicle. The temperature measuring device is preferably disposed in a flow path of an air flow. According to the invention, the air flow from the ambient air of the vehicle can be generated both by speed-dependent wind resistance of the vehicle and/or wind and also by a compressor unit disposed in the flow path.
The compressor unit is preferably configured as a fan or a blower. The compressor unit may be disposed in the flow path in front of or behind the temperature measuring device. Particularly, preferably the compressor unit and the temperature measuring device are disposed in the immediate proximity of one another, so that the compressor unit and the temperature measuring device can be integrated in a common housing.
The compressor unit is preferably configured to detect a current flow rate of the air flow and/or a current speed of the compressor unit. Alternatively, an additional measuring device for measuring the flow rate of the air flow can also be disposed in the flow path. The temperature measuring device can also be coupled to a control unit for the compressor unit. Based upon a detected flow rate and/or based upon the current speed and a current power consumption of the compressor unit, the control unit may be configured to control the output of the compressor unit. Thus, it is possible to adjust a predetermined air flow for the temperature measuring device.
The control unit is preferably configured to disconnect the compressor or set it to idle operation unit at a predetermined flow rate or vehicle speed. This protects the compressor unit and avoids unnecessary power consumption by the compressor unit.
A second aspect of the invention relates to a control device for providing a temperature-compensated sound velocity value, preferably for a temperature-compensated distance measurement with ultrasound. For this purpose the control device is coupled to a temperature measuring device according to the invention, and is configured to determine a current sound velocity based upon a temperature of the ambient air currently registered by the temperature measuring device.
A third aspect of the invention relates to a motor vehicle with a temperature measuring device according to the invention, or with a control device according to the invention for providing a temperature-compensated sound velocity value, preferably for a temperature-compensated distance measurement with ultrasound.
A fourth aspect of the invention relates to a method for measurement of the external temperature of the ambient air in a vehicle with a temperature measuring unit. The method comprises the following steps: generating an air flow from a speed-dependent wind resistance of the vehicle and/or wind and additionally by means of a compressor unit; and supplying the temperature measuring unit with the air flow. The method may preferably also include: controlling the output of the compressor unit based on a current flow rate of the air flow and/or a current speed of the vehicle.
A fifth aspect of the invention relates to a method for providing a temperature-compensated sound velocity value, preferably for a temperature-compensated distance measurement with ultrasound, based on one of the above-mentioned methods according to the invention for measurement of the current external temperature of the ambient air in the vehicle. An external temperature of the ambient air currently measured by the temperature measuring unit is used, as soon as a predetermined flow rate of the air flow is present, for calculation of the current sound velocity in the ambient air.
Further advantages, features and details of the invention are apparent from the following description in which embodiments of the invention are described in detail with reference to the drawings. In this case the features mentioned in the claims and in the description may in each case be essential to the invention individually or in any combination. Likewise the features referred to above and the further features set out here can each be used alone in any combinations. Some structural parts or components which are functionally similar or identical are provided with the same references. The expressions used in the description of the embodiments “left”, “right”, “top” and “bottom” relate to the drawings in an orientation with normally readable figure designation or normally readable reference signs. The embodiments shown and described should not be understood as definitive, but rather as examples for explanation of the invention. The detailed description serves for information for the person skilled in the art, therefore known circuits, structures and methods are not shown or explained in detail in the description, in order not to make it difficult to understand the present description not to making.